1. Field of the Invention
The present invention relates to continuous wave electron-beam accelerators and continuous wave electron-beam accelerating methods thereof, and in particular, to continuous wave electron-beam accelerators for accelerating high intensity continuous wave electron beams particularly sludge for use in food irradiation, irradiation for quarantine, sludge processing, drainage processing, medical sterilization, the generation of low energy positrons, etc., and continuous wave electron-beam accelerating methods thereof.
2. Description of the Related Art
FIG. 10 shows a conventional electron-beam accelerator as described in, for example, Takahashi and Yamada, xe2x80x9cDevelopment of Small-sized Synchrotron Radiation Source xe2x80x98AURORAxe2x80x99xe2x80x9d, Sumitomo Jukikai (Heavy Industries) Giho (Technical Report), Vol. 39, No. 116, 1991, pp. 2-10. This type of electron-beam accelerator is called a xe2x80x9crace-track microtronxe2x80x9d. FIG. 10 shows an electron gun 111, an injection electromagnet 112, a radio frequency cavity (linac) 113, bending electromagnets 114, and electron beam orbits 115.
The operation of the conventional electron-beam accelerator is described below.
An electron beam is generated by the electron gun 111. The generated electron beam is a pulsed beam having a frequency of several hertz to several hundred hertz and a pulse width of ten nanoseconds to several microseconds.
The generated electron beam is injected into the electron-beam accelerator by the injection electromagnet 112. In the electron-beam accelerator, the electron beam is accelerated whenever it passes through the radio frequency cavity 113 while passing along the electron beam orbits 115. The electron-beam accelerator accelerates the electron beam by mainly using an S-band radio-frequency electric field (approximately 2.8 GHZ). When the electron beam passes through the radio frequency cavity 113 once, it usually obtains an energy of approximately 5 MeV. In order to form the electron beam orbits 115, the bending electromagnets 114 are disposed across the radio frequency cavity 113.
In the electron-beam accelerator, the acceleration phase of the electron beam each time it circumferentially passes through the radio frequency cavity 113 is uniquely determined by an expression of the relationship between an acceleration voltage in the radio frequency cavity 113 and the magnetic field strength of the bending magnets 114. Accordingly, to enable the acceleration of the electron beam up to a high energy level, two conditions must be satisfied: (1) energy gain obtained when the electron beam passes through the radio frequency cavity 113 is close to a multiple of the electron rest energy (approximately 511 keV), and (2) the speed of the electron beam is close to the speed of light.
When the injection energy of the electron beam is low, the speed of the electron beam is much smaller than the speed of light (for example, when the injection energy is 80 keV, the electron beam speed is approximately half of the speed of light), the above conditions do not hold. In addition, when the energy gain obtained when the electron beam passes through the radio frequency cavity 113 is small, the number of circumferential passes of the electron beam until its speed approaches the speed of light increases, which causes a problem in that acceleration is difficult since a shift from the acceleration phase increases during the circumferential passes. Accordingly, the conventional electron-beam accelerator must be operated using parameters in which, by raising the acceleration voltage of the radio frequency cavity 113, the electron beam speed almost reaches the speed of light when the electron beam is allowed to pass through the radio frequency cavity 113 once or slightly more.
In order to increase the acceleration voltage per unit length, the frequency of a radio frequency electric field applied to the radio frequency cavity 113 must be increased to approximately 1 GHz to 3 GHz. In order to increase the acceleration voltage of the radio frequency cavity 113 when the frequency of the radio frequency electric field is smaller than this value, the size of the radio frequency cavity 113 must be increased. This is because, while the electron beam passes through the radio frequency cavity 113, it has a deceleration phase and can hardly be accelerated since a shift of the phase of the electron beam from the radio frequency acceleration electric field rapidly increases.
A radio frequency cavity having a radio frequency of 1 GHz to 3 GHz causes a problem in that it is difficult to accelerate a continuous wave electron beam having a large average current since the size of the radio frequency cavity is inevitably small and it is difficult to remove heat generated when high power is supplied. Therefore, it is difficult to apply electron-beam accelerators having a radio frequency cavity of this type to purposes requiring a high intensity continuous wave electron beam, such as food irradiation, irradiation for quarantine, sludge processing, drainage processing, medical sterilization, and generation of low energy positrons.
In the conventional electron-beam accelerator, the microtron acceleration condition must be satisfied such that the energy gain for each circumferential pass of the electron beam must be approximately a multiple of the electron rest energy (approximately 511 keV). Thus, a problem occurs in that electrical efficiency cannot be increased due to parameter limitation.
Accordingly, the present invention is made for solving the foregoing problems. A first object of the present invention is to provide a continuous wave electron-beam accelerator for accelerating an electron beam having a large average current and a continuous wave accelerating method thereof.
A second object of the present invention is to provide a continuous wave electron-beam accelerator in which an electron beam is accelerated without satisfying the condition that the energy gain for each circumferential pass of an electron beam must be approximately a multiple of the electron rest energy, which is required in microtron acceleration and in which parameters have more degrees of freedom, resulting in an increase in electrical efficiency, and a continuous wave electron-beam accelerating method thereof.
According to an aspect of the present invention, a continuous wave electron-beam accelerator includes an electron-beam generating unit for generating a continuous wave electron beam, an electron-beam accelerating unit for accelerating the continuous wave electron beam, a first electron-beam bending unit that is provided close to one end of the electron-beam accelerating unit and that bends the accelerated continuous wave electron beam, and a second electron-beam bending unit that is provided close to the other end of the electron-beam accelerating unit and that bends the accelerated continuous wave electron beam. Each of the first electron-beam bending unit and the second electron-beam bending unit includes a first bending electromagnet having a surface opposed to one side of the electron-beam accelerating unit, a second bending electromagnet and a third bending electromagnet which are discretely provided opposing another surface of the first bending electromagnet. The first bending electromagnet is made of a reverse bending electromagnet having a polarity opposite to that of the second bending electromagnet or the third bending electromagnet. The second bending electromagnet has a polarity identical to that of the third bending electromagnet, and has a first magnetic field strength different from that of the third bending electromagnet. The third bending electromagnet has a polarity identical to that of the second bending electromagnet, and has a second magnetic field strength different from that of the second bending electromagnet.
The present invention also provides a continuous wave electron-beam accelerator including an electron-beam generating unit for generating a continuous wave electron beam, an electron-beam accelerating unit for accelerating the continuous wave electron beam, and an electron-beam bending unit for bending the accelerated continuous wave electron beam. The electron-beam bending unit includes a first electron-beam bending unit that is provided close to one end of the electron-beam accelerating unit and that bends the accelerated continuous wave electron beam, a second electron-beam bending unit that is provided close to the other end of the electron-beam accelerating unit and that bends the accelerated continuous wave electron beam, and a third electron-beam bending unit that is provided between the first electron-beam bending unit and the second electron-beam bending unit at a straight portion opposed to the electron-beam accelerating unit, and that generates dipole magnetic fields for adjusting the length of the circumferential path of the continuous wave electron beam when the continuous wave electron beam passes through the magnetic fields.
According to the above-described continuous wave electron-beam accelerators, it is possible to select, for the electron-beam accelerating unit, a radio-frequency electric field having a low acceleration frequency. This enables the acceleration of a continuous wave electron beam having a large average current.
In addition, without satisfying the condition that the energy gain for each circumferential pass must be approximately a multiple of the electron rest energy, which is essential in the microtron acceleration, the continuous wave electron beam can be accelerated, and the parameter has more degrees of freedom. As a result, the electrical efficiency can be increased. Moreover, the loss caused by the wall in the electron-beam accelerating unit can be decreased, which increases the electrical efficiency.
According to another aspect of the present invention, a continuous wave electron-beam accelerating method for a continuous wave electron-beam accelerator includes an electron-beam generating unit for generating a continuous wave electron beam, an electron-beam accelerating unit for accelerating the continuous wave electron beam, a first electron-beam bending unit that is provided close to one end of the electron-beam accelerating unit and that bends the accelerated continuous wave electron beam, and second electron-beam bending unit that is provided close to the other end of the electron-beam accelerating unit and that bends the accelerated continuous wave electron beam. The continuous wave electron-beam accelerating method includes the steps of (a) adjusting the acceleration phase of the continuous wave electron beam, which is injected into the electron-beam accelerating unit, by adjusting the difference between the phase of the continuous wave electron beam in the electron-beam generating unit and the phase of an acceleration electric field in the electron-beam accelerating unit, (b) adjusting the acceleration phase of the continuous wave electron beam, which is injected into the electron-beam accelerating unit, by adjusting the distance between the electron-beam accelerating unit and the first electron-beam bending unit, (c) adjusting the acceleration phase of the continuous wave electron beam, which is injected into the electron-beam accelerating unit, by adjusting the distance between the first electron-beam bending unit and the second electron-beam bending unit, and (d) adjusting the acceleration phase of the continuous wave electron beam, which is injected into the electron-beam accelerating unit, by adjusting a ratio between the magnetic field strengths of identical-polarity bending electromagnets provided in the first electron-beam bending unit and the second electron-beam bending unit, and the bending angles thereof.
The present invention also provides a continuous wave accelerating method for a continuous wave electron-beam accelerator including an electron-beam generating unit for generating a continuous wave electron beam, an electron-beam accelerating unit for accelerating the continuous wave electron beam, a first electron-beam bending unit that is provided close to one end of the electron-beam accelerating unit and that bends the accelerated continuous wave electron beam, a second electron-beam bending unit that is provided close to the other end of the electron-beam accelerating unit and that bends the accelerated continuous wave electron beam, and a third electron-beam bending unit that is provided between the first electron-beam bending unit and the second electron-beam bending unit so as to be opposed to the electron-beam accelerating unit, and that generates dipole magnetic fields for adjusting the length of the circumferential path of the continuous wave electron beam which passes through the magnetic fields. The continuous wave accelerating method includes the steps of (a) adjusting the acceleration phase of the continuous wave electron beam, which is injected into the electron-beam accelerating unit, by adjusting the difference between the phase of the continuous wave electron beam in the electron-beam generating unit and the phase of an acceleration electric field in the electron-beam accelerating unit, (b) adjusting the acceleration phase of the continuous wave electron beam, which is injected into the electron-beam accelerating unit, by adjusting the distance between the electron-beam accelerating unit and the first electron-beam bending unit, (c) adjusting the acceleration phase of the continuous wave electron beam, which is injected into the electron-beam accelerating unit, by adjusting the distance between the first electron-beam bending unit and the second electron-beam bending unit, and (d) adjusting the acceleration phase of the continuous wave electron beam, which is injected into the electron-beam accelerating unit, by adjusting the length of the path of the continuous wave electron beam each time the continuous wave electron beam circumferentially passes.
According to the above-described continuous wave accelerating methods, without satisfying the condition that the energy gain for each circumferential pass must be approximately a multiple of the electron rest energy, which is essential in the microtron acceleration, a continuous wave electron beam can be accelerated.
The foregoing and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken into conjunction with the accompanying drawings.